First tube amp(6505+) question

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by slothrop, Feb 24, 2012.

  1. slothrop

    slothrop Member

    May 12, 2011
    So I picked up a used 6505+ the other day which sounds pretty good but there's a constant noise and I'm hoping someone can tell me if it's just tubes or a bigger problem.

    I'm not sure if it would be considered hiss or hum it's more like a constant shhhhhhhhh sound which happens when the volume is over 1 and the gain set to anything(even 0), happens when guitar is plugged in or not, the amp is plugged into a house outlet and I have tried several. I can hear it on the lead and rhythm channels but disappears when I turn off 'crunch' on the rhythm channel.

    I also tried hooking up an hd500 using the 4 cable method and when I use amp sims there is no extra noise and everything sounds good. When I use a preset using the 6505's preamp I get the noise.

    I'm going to bring it to an amp tech before returning it but any advice appreciated.
  2. RussB

    RussB low rent hobbyist Silver Supporting Member

    Jul 3, 2004
    My son plays 5150's and has had several.

    With any used amp, SOP is to replace all the tubes. In that series of amps, I like the Chinese 9th gen 12AX7's that are chosen for low noise (Mike @ KCA NOS tubes has them for a fair price) Pre-amp tubes are important in those amps due to their high noise floor, even with good tubes. Avoid the Tung_Sol 12AX7's, and they can & will fail in cathode follower positions. Position 1, 2 and 3 will have the most impact on tone, and subsequently, noise and hiss

    I find them less sensitive to power tubes because the pre-amp generates all the crunch and distortion.

    Here's the pre-amp tube layout,

    5150 II/6505+

    If you look at the rear of the amp, the preamp tubes are again on the right side.
    Looking from right to left, the tubes are arranged as follows:
    #1 (far right) the rhythm channel
    #2 is shared by the rhythm and lead channel (input stage)
    #3 and 4- lead channel
    #5 effects loop driver
    #6 phase splitter
  3. slothrop

    slothrop Member

    May 12, 2011
    Thanks for the info Russ, I should have mentioned that this is a 1x12 combo with 5 preamp tubes and I guess the sound is more of a white noise.

    Since I don't have any tubes lying around I brought it back to the store and they swapped all the tubes, the only one that made a difference was the 2nd from right(looking at the back) it helped a bit but there was still some noise.

    They're sending it to their amp guy who I guess is one of the best in Mpls, they'll have him fix it at no charge to me which is fine since I got a great deal on this amp to begin with.

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